1. Field of the Invention
The present invention relates to a vehicle mounted dedicated short-range communication ((hereinafter referred to as DSRC) apparatus which receives transmission data from a ground or on-road device, and outputs an ASK modulated signal and a QPSK modulated signal by using a DSRC system in an intelligent transport system (hereinafter referred to as ITS) including ground devices. More specifically, it relates to a vehicle mounted DSRC apparatus which is capable of achieving reduction in its cost and size by sharing an ASK filter circuit with a QPSK filter circuit.
2. Description of the Prior Art
Conventionally, a DSRC system in association with an ITS has been known well as a system by which communicates are carried out only within a limited range on a road by using radio waves of a microwave band.
Such a kind of DSRC system is an effective system in which radio communications are carried out between a ground device arranged on a road and an on-vehicle device installed on a vehicle, whereby various services such as toll collection, traffic and road information services, etc., can be provided by mutually transferring various types of data between the ground device and the on-vehicle device, thus benefiting not only drivers but also administrators of roads, parking lots and so on.
As systems using DSRC, there have been thought various kinds of applications such as electronic toll collection (ETC) systems in expressways, charge collection systems in gas stations and drive throughs, traffic information services and so on.
Moreover, according to the DSRC standard for example, two modulation systems, ASK (Amplitude Shift Keying) modulation and QPSK (Quadrature Phase Shift Keying) modulation, are defined as modulation systems usable for various applications.
The above-mentioned ETC system has already been provided as an example of ASK modulation applications, and a charge settlement system for drive-throughs or gas stations is planned as an example of QPSK modulation applications.
FIG. 10 is a block diagram of a known vehicle mounted DSRC apparatus, showing the case where provision is made for an ASK modulation part 1 for generating an ASK modulated signal and a QPSK modulation part 2 for generating a QPSK modulated signal.
In FIG. 10, the vehicle mounted DSRC apparatus includes, in addition to the ASK modulation part 1 and the QPSK modulation part 2, a power amplifier (PA) 30 which amplifies modulation data from the modulation parts 1 and 2, a power supply 40 which supplies electric power to the modulation parts 1 and 2, respectively, a control part 100 which transmits and receives various data, and controls the modulation parts 1 and 2, respectively, an antenna 101 for data transmission and reception, and a transmission and reception switch 102 which is interposed between the antenna 101 and the control part 100, and between a power amplifier (PA) 30 and the control part 100.
A low noise amplifier (LNA) 103, a mixer (hereinafter referred to as “MIX”) 104 and a demodulator 105 are interposed between the transmission and reception switch 102 and the control part 100.
In addition, connected to an output terminal of the control part 100 is an oscillation part 106 which generates a signaling frequency or frequency signal input to the respective modulation parts 1, 2 and the MIX 104.
The transmission and reception switch 102 performs the switching of transmission and reception of various data signals, so that radio waves can be transmitted and received through the antenna 101.
The low noise amplifier 103 serves to amplify received radio signals.
The MIX 104 converts a received frequency into a low frequency signal which is easy to handle, by using a frequency difference between the signaling frequency from the oscillation part 106 and the received frequency.
The demodulation part 105 detects the transmission data of the low frequency output from the MIX 104 and outputs it to the control part 100.
The ASK modulation part 1 includes an ASK data generation part 11 for generating transmission data for ASK use (i.e., ASK data), a band limiting filter 12 which constitutes a low pass filter (LPF) for ASK use (i.e., ASK low pass filter), and an ASK modulator 13 for ASK modulating the ASK data from the band limiting filter 12.
The ASK modulator 13 is operated by electric power supplied from the power supply 40.
The band limiting filter 12 performs the band limiting of transmission data (digital data) output by the ASK data generation part 11.
The ASK modulation data signal output from the ASK modulator 13 is amplified to a desired level by means of the power amplifier 30, and then sent to the transmission and reception switch 102 through which it is passed to the antenna 101 and is transmitted therefrom as a transmission microwave.
On the other hand, the QPSK modulation part 2 includes an I data generation part 21 for generating I channel data (hereinafter referred to as “I data”) for QPSK use, a Q data generation part 22 for generating Q channel data (hereinafter referred to as “Q data”) for QPSK use, a first band limiting filter (hereinafter simply referred to as “band limiting filter”) 23 for limiting the band of the I data, and a second band limiting filter (hereinafter simply referred to as “band limiting filter”) 24 for limiting the band of the Q data.
Also, the QPSK modulation part 2 further includes a low pass filter (LPF ) 25 for rejecting or removing harmonic components from the I data which has passed through the band limiting filter 23, a low pass filter (LPF) 26 for rejecting or removing harmonic components from the Q data which has passed through the band limiting filter 24, and a QPSK modulator 27 for performing QPSK modulation based on the QPSK data (I data and Q data) output from the low pass filters 25, 26.
The QPSK modulator 27 is operated by electric power supplied from the power supply 40.
The ASK data generation part 11, the I data generation part 21 and the Q data generation part 22 generate ASK data or QPSK data under the control of the control part 100, but do not generate ASK data and QPSK data at the same time.
The signaling frequency output from the oscillation part 106 under the control of the control part 100 is input to the ASK modulator 13 and the QPSK modulator 27.
The QPSK modulation data signal output from the QPSK modulator 27 is amplified to a desired level by the power amplifier 30, and then sent to the transmission and reception switch 102 through which it is passed to the antenna 101 and transmitted therefrom as a transmission microwave.
The power amplifier 30 is commonly used for amplifying both the ASK modulated data and the QPSK modulated data.
In general, the band limiting filters 23, 24 in the QPSK modulation part 2 are each constituted by a DA converter, and hence the output signals of the band limiting filters 23, 24 include harmonic components (i.e., sampling noise) due to DA sampling. To remove the harmonic components, provision is made for low pass filters (LPFS) 25 and 26.
FIG. 11 is a waveform view showing the output waveforms of the respective parts in FIG. 10, including the output waveform (rectangular wave) of each of ASK data, I data and Q data, the output waveform (sine wave including harmonic components) of each of the band limiting filters 23, 24, and the output waveform (sine wave) of each of the band limiting filter 12 and the low pass filters 25, 26.
FIG. 12 is a circuit diagram showing an example of the configuration of the band limiting filter 12 in FIG. 10.
In FIG. 12, the band limiting filter 12 is constituted by a plurality of serially connected low pass filters (LPFs), i.e., multistage LC (reactor and capacitor) circuits.
FIG. 13 is a circuit diagram showing an example of the configuration of the low pass filter (LPF) 25 (or LPF 26) in FIG. 10.
In FIG. 13, the low pass filter 25 is constituted by a single LC circuit or a small number of LC circuits.
In general, in the vehicle mounted DSRC apparatus shown in FIG. 10, the band limiting filters 23, 24 in the QPSK modulation part 2 perform sampling at frequencies sufficiently higher than that of the transmission data. Accordingly, the low pass filters 25, 26 for sampling noise rejection can be implemented by a low-stage filter configuration as shown in FIG. 13.
In contrast to this, the band limiting filter 12 in the ASK modulation part 1 treats the rectangular wave of the ASK data (see FIG. 11) generated by the ASK data generation part 11. Thus, in order to limit the wide-band digital signal to a desired band, it becomes necessary to employ a high-stage filter configuration as shown in FIG. 12.
In addition, in the known vehicle mounted DSRC apparatus as shown in FIG. 10, it is necessary to concurrently arrange individual filter circuit configurations in the ASK modulation part 1 and in the QPSK modulation part 2, respectively.
With the known vehicle mounted DSRC apparatus as described above, the filter circuit configurations are individually provided in the ASK modulation part 1 and in the QPSK modulation part 2, respectively, which together constitute a transmission circuit, and hence the size of the circuit becomes large. As a result, there arise the following problems. That is, the cost of manufacture is increased, thus reducing user's purchasing interest or desire. Besides, the consumption of current increases, thereby increasing the calorific value of the apparatus.
In addition, there is another problem in that the overall shape of the known vehicle mounted DSRC apparatus becomes large, thus resulting in deteriorated installability on a vehicle, and in the worst case, it becomes impossible to install the apparatus in a user's desired installation place.